Currently, data communication by wireless LAN (Local Area Network) is popular and is widely used in communication between electronic devices. The IEEE (The Institute of Electrical and Electronics Engineers) 802.11 standard, as a communication standard for wireless LAN, includes IEEE 802.11a/11b/11g/11n/11ac and the like. Among them, IEEE 802.11ac is attracting attention as the communication standard for wireless LAN of the next generation.
Electrical devices for performing data communication in wireless LAN in accordance with the IEEE 802.11ac standard, for example, is provided with a front-end circuit. The front-end circuit is connected to an antenna and configured to amplify both a transmission signal and a reception signal (PTD 1: WO 2007/129716).
FIG. 18 is a circuit block diagram illustrating the configuration of a conventional front-end circuit. With reference to FIG. 18, front-end circuit 100 is provided with a PA (Power Amplifier) 101a, an LNA (Low Noise Amplifier) 102a, a bypass circuit 102b, an antenna switch 103, a transmission terminal TX, a reception terminal RX, and an antenna terminal ANT.
PA 101a and LNA 102a each is an amplifier circuit configured to amplify a weak input signal and output the signal after amplification. PA 101a amplifies a signal received by transmission terminal TX. The signal amplified by PA 101a is output as a transmission signal. LNA 102a amplifies a reception signal received by antenna terminal ANT.
Bypass circuit 102b forms a bypass from a start point A provided between antenna switch 103 and the input terminal of LNA 102a to an end point B provided between the output terminal of LNA 102a and reception terminal RX. Bypass circuit 102b includes a switch SW which is a switch of SPST (Single Pole Single Throw) type. In front-end circuit 100, switch SW is controlled on or off to select whether to amplify the signal reaching start point A with LNA 102a or make the signal bypass LNA 102a via bypass circuit 102b. 
Antenna switch 103 is a switch of SPDT (Single Pole Dual Throw) type. Antenna switch 103 includes a terminal TA, a terminal TT and a terminal TR. Terminal TA is connected to antenna terminal ANT. Terminal TT is electrically connected to transmission terminal TX. Terminal TR is electrically connected to reception terminal RX. Antenna switch 103 is switched to connect terminal TA to terminal TT in the transmission operation of front-end circuit 100 and connect terminal TA to terminal TR in the reception operation thereof.
First, the transmission operation of front-end circuit 100 will be described. Antenna switch 103 is switched to connect terminal TA to terminal TT. An RFIC (radio frequency integrated circuit) 200 outputs a signal to transmission terminal TX. The signal received by transmission terminal TX is amplified by PA 101a and is transferred from terminal TT to terminal TA TT inside antenna switch 103 as a transmission signal. After the transmission signal reaches antenna terminal ANT, it is transmitted out from antenna 300.
In the high frequency circuit disclosed in PTD 2 (Japanese Patent Laying-Open No. 2009-33598), a terminal is added to the antenna switch, and the signal strength of a reception signal input to LNA is increased or decreased by using the isolation feature of the antenna switch. With this configuration, the bypass circuit is no longer required.
In comparison to the conventional IEEE 802.11a/11b/11g/11n or the like, IEEE 802.11ac requires PA 101a to have a higher linearity. As the linearity of PA 101a is increased, the distortion of the transmission signal occurred upon amplification will be reduced. As a result, the modulation accuracy of the transmission signal will be improved. Generally, increasing the bypass voltage of PA 101a will improve the linearity of PA 101a. However, the attempt of improving the linearity of PA 101a simply by increasing the bypass voltage of PA 101a will increase power consumption in PA 101a. Therefore, the battery life of an electronic device provided with front-end circuit 100 will become shorter. Thereby, a technique called digital pre-distortion (hereinafter, referred to as DPD) in IEEE 802.11ac is generally used to cope with such problem.
FIG. 19 is a block diagram illustrating the concept of DPD. With reference to FIG. 19, in DPD, a distortion compensation circuit 104 and a coupler 105 are employed in addition to PA 101a. An input signal Sin is a transmission signal before being amplified by PA 101a. An output signal Sout is a transmission signal after input signal Sin is amplified by PA 101a. 
In the case where the signal strength of input signal Sin is high, when input signal Sin is amplified by PA 101a, the distortion of the transmission signal will become great. To cope with this problem, distortion compensation circuit 104 is provided before PA 101a. A part of output signal Sout is branched by coupler 105, and the signal branched by coupler 105 is received by distortion compensation circuit 104. The branched signal is called a loopback signal S_loop. Distortion compensation circuit 104 generates a signal distorted in a direction opposite to the distortion occurred in loopback signal S_loop, combines the signal with input signal Sin, and outputs it as a distortion compensation signal Sp. Thus, the use of DPD makes it possible to obtain a transmission signal with a reduced distortion while inhibiting the power consumption from increasing.
However, typically, a coupler requires a transfer line to have a line length of λ/4 (λ: wavelength). Thus, in order to provide coupler 105 in the transfer line on a mounting substrate, the substrate is required to have a greater area. Nowadays, the presence of chip components of coupler 105 makes it possible to reduce the area of the substrate. However, in this case, along with the addition of new chip components, the material cost will increase.
FIG. 20 is a circuit block diagram illustrating the configuration and a transfer line for transferring signals of a conventional front-end circuit which uses the isolation feature of an antenna switch to branch a loopback signal. Note that a front-end circuit 110 illustrated in FIG. 20 is different from front-end circuit 100 illustrated in FIG. 18 in that front-end circuit 110 includes an antenna switch 113 to replace antenna switch 103. Since the other portions of front-end circuit 110 are identical to the corresponding portions of front-end circuit 100 in configuration, they will be denoted by the same reference signs and the description thereof will not be repeated.
A distortion compensation circuit 204 is provided in RFIC 200. A signal (indicated by a dashed line) output from distortion compensation circuit 204 is amplified by PA 101a, and reaches antenna switch 113. Since terminal TT and terminal TR are electrically insulated in antenna switch 113, when a transmission signal is transferred from terminal TT to terminal TA, most of the transmission signal will reach terminal TA. However, since the isolation between the transfer line for transferring the transmission signal from terminal TT to terminal TA and the transfer line for transferring the reception signal from terminal TT to terminal TR has a limit, a part of the transmission signal will leak to the transfer line for transferring the reception signal as the loopback signal (indicated by a dotted line). The leaked loopback signal bypasses LNA 102a via bypass circuit 102b, and is output to distortion compensation circuit 204 from reception terminal RX.
As described in the above, by using the isolation feature of antenna switch 113, it is possible to branch a loopback signal without adding coupler 105 and output the loopback signal to distortion compensation circuit 204.
Next, referring again to FIG. 18, the reception operation of front-end circuit 100 will be described. Antenna switch 103 is switched to connect terminal TA to terminal TR. The reception signal received by antenna 300 is output to antenna terminal ANT, and the reception signal received by antenna terminal ANT is transferred from terminal TA to terminal TR inside antenna switch 103. In order to perform a signal treatment of the reception signal in the subsequent RFIC 200, the reception signal should be confined within a range of appropriate signal strengths. Therefore, as the reception signal reaches start point A, the transfer line is switched in response to the signal strength.
In the case where the signal strength of the reception signal is low (in the case where the radio wave condition is poor), LNA 102a is turned on and switch SW is turned off. Thus, the reception signal is amplified by LNA 102a. The reception signal amplified by LNA 102a is output to RFIC 200 from reception terminal RX.
On the other hand, in the case where the signal strength of the reception signal is high (in the case where the radio wave condition is good), LNA 102a is turned off and switch SW is turned on. Thus, the reception signal bypasses LNA 102a via bypass circuit 102b. Thereby, the reception signal will not be amplified by LNA 102a. The reason therefor is that if the reception signal of a high signal strength is amplified by LNA 102a, the distortion will be caused to occur in the reception signal by LNA 102a and RFIC 200. The reception signal bypassed LNA 102a via bypass circuit 102b is output to RFIC 200 from reception terminal RX.
However, in the case where the signal strength of the reception signal is high, if the reception signal is simply made to bypass LNA 102a without being amplified by LNA 102a, the reception signal will not be constrained within the range of appropriate signal strengths. In order to meet the specific requirements prescribed in IEEE 802.11ac, generally it is necessary to attenuate the reception signal by about −15 dB to −5 dB.